An occupancy sensor is designed to detect the presence of a person(s) in a room, usually in order to determine whether various electrically powered loads in that room (for example, lights, ventilation, and the like) should be turned on or not. This is of particular advantage to institutions that have occupants who are not directly responsible for paying for the electricity they consume, since these people often do not exercise diligence in regularly turning off electrically powered loads, such as lights, ventilation, and the like, when they leave a room. Occupancy sensors may therefore conserve a great deal of energy. This has led many businesses to purchase them voluntarily; it has also resulted in laws in certain states mandating the use of occupancy sensors in large areas as an environmental conservation measure.
The two most prevalent types of occupancy sensors used with automatic wall switches, either singularly or in combination with one another, are passive infrared and active ultrasonic devices.
Generally, a passive infrared (“PIR”) sensor will turn on the load whenever it detects a moving or newly apparent heat source. Passive infrared occupancy detection technology allows continuous detection of moving objects that emit infrared energy. This method of occupancy detection is also quite sensitive even though it is based on passive sensing of moving sources of infrared energy.
An active ultrasonic sensor emits vibrations at frequencies of 25 kHz or higher and listens to the return echoes; if it detects a significant Doppler shift, indicating the presence of a moving body, then it turns the load on. Either detector will turn the load back off after a certain interval of no motion sensed, usually three to sixty minutes as determined by the user. The motion sensitivity of the device is usually also set by the user.
More specifically, active ultrasonic acoustic Doppler occupancy detection technology allows continuous detection of moving objects that reflect ultrasonic acoustic energy. For example, currently available light switches or the like used in offices emit an ultrasonic wave into a room and detect motion of persons by sensing a Doppler-shift in the reflected ultrasonic wave. The Doppler-shift in the reflected wave is caused by persons moving within the room. This method of occupancy detection is highly sensitive since it is based on an active source of ultrasonic acoustic energy. An apparatus and method of this type are disclosed in U.S. Pat. No. 5,640,143, to Myron et al (assigned to the same assignee as the present invention), the entire disclosure of which is incorporated hereby by reference.
Each of these types of sensors is not without disadvantage. For example, PIR sensors require a lens. The lens has an exposed front wall which allows transmission of infrared energy to detect occupancy. The front wall is typically arranged in close proximity to manual override switches. Consequently, in high-abuse applications such as schools and offices, the lens is continuously poked and prodded during attempts to activate the manual override switch. For example, the lens is often damaged due to acts of vandalism. Thus, the structural integrity of the lens is often compromised and requires replacement.
Ultrasonic sensors utilize transducers to emit and receive sonic energy. Typically, to minimize the size of the device, the transducers are mounted directly onto the circuit board. The transducers are arranged perpendicular to the circuit board and define an axis. The transducers send and receive a sensitivity pattern. The sensitivity pattern is strongest on the transducer axis. The sensitivity pattern weakens away from the transducer axis. Therefore, the resultant composite sensitivity pattern of the sender and receiver transducers is considerably greater along the transducer axis, but, considerably less to the sides. This is undesirable, since the sensor pattern should have uniform sensitivity to the sides of the transducer axis to effectively cover the entire controlled space.
To protect the ultrasonic transducers, a grille is typically placed in front of the transducers. The grille is typically designed with openings to allow suitable passage of acoustic energy through the grille. When servicing the connected lighting load, power should be disconnected from the load. Circuit interruption at the breaker is the preferable way to disconnect power; however, electricians often use a manual wall switch to disconnect power to a circuit. An automatic occupancy sensor wall switch may subsequently re-energize the load, thus, presenting a problem. Consequently regulatory bodies often require a switch in the occupancy sensor to prohibit the sensor from energizing the load. This is commonly referred to as an “air-gap” switch, indicating that it is composed of metal contacts separated by air.
The air-gap switch in an occupancy sensor is typically hidden and requires disassembly of the switch cover plate for access. After completing service on the lighting load, an electrician should close the air-gap switch, but, often this step is forgotten. Consequently, the switch cover plate is reassembled with the air gap switch left in the open position. This necessitates a return to the switch and subsequent disassembly and reassembly of the cover plate to close the switch. Thus, valuable time is wasted.
Accordingly, in order to address these disadvantages, there have been various additional attempts to provide improved occupancy sensors. Examples of such occupancy sensors are disclosed in U.S. Pat. Nos. 6,798,341 to Eckel et al.; U.S. Pat. No. 6,587,049 to Thacker; U.S. Pat. No. 6,480,103 to McCarthy et al.; U.S. Pat. No. 6,222,191 to Myron et al.; U.S. Pat. No. 6,150,943 to Lehman et al.; U.S. Pat. No. 6,082,894 to Batko et al.; U.S. Pat. No. 6,049,281 to Osterweil; U.S. Pat. No. 5,973,594 to Baldwin; U.S. Pat. No. 5,861,806 to Vories et al.; U.S. Pat. No. 5,703,368 to Tomooka et al.; U.S. Pat. No. 5,394,035 to Elwell; U.S. Pat. No. 5,392,631 to Elwell; U.S. Pat. No. 5,363,688 to Elwell; U.S. Pat. No. 5,319,283 to Elwell; U.S. Pat. No. 5,293,097 to Elwell; U.S. Pat. No. 5,281,961 to Elwell; U.S. Pat. No. 5,142,199 to Elwell; U.S. Pat. No. 4,841,285 to Laut; U.S. Pat. No. 4,751,399 to Koehring et al.; U.S. Pat. No. 4,703,171 to Kahl; U.S. Pat. No. 4,678,985 to Moski; U.S. Pat. No. 4,418,337 to Bader; U.S. Pat. No. 4,057,794 to Grossfield; and U.S. Pat. No. 2,096,839 to Barlow. Although some of the features of those occupancy sensor assemblies ease the disadvantages described above, a continuing need exists for an improved occupancy sensor assembly which facilitates maintenance of the sensor assembly, enhances effectiveness of a ultrasonic sensor, and minimizes damage to the assembly in high abuse applications.